The present invention relates to a DA converter incorporated in liquid crystal driving and other devices, and further relates to a liquid crystal driving device incorporating such a DA converter.
A DA (digital-to-analog) converter converts externally inputted digital signals to analog signals. For example, in a liquid crystal driving device of an active matrix liquid crystal display device, a DA converter is used to convert externally inputted digital signals, which constitute display data, to analog signals, and to send the analog signals to a liquid crystal display section. Some DA converters of this kind include an OP-amplifier composed of MOS transistors.
FIGS. 10 and 11 show a detailed structure of a DA converter 100 incorporated in a liquid crystal driving device as mentioned above, which converts display data in the form of digital signals to analog voltages for outputs.
The DA converter 100 is made up of a selector circuit 106 [corresponding to a selector circuit 16 of a DA converter 10 of the present invention (FIG. 9)], which is a DA conversion circuit; a voltage follower circuit 107 (corresponding to a voltage follower circuit 17 in FIG. 9), which is an output circuit; and a reference voltage generating circuit 108 (corresponding to a reference voltage generating circuit 18 in FIG. 9). FIG. 10 shows an example of a structure of a DA converter incorporated in a 64-level gradation liquid crystal driving which outputs 64 analog voltages in accordance with 6-bit digital signals (Bit 5 to Bit 0). FIG. 11 is an enlarged view of the section from V48 to V64 of the reference voltage generating circuit 108 and the selector circuit 106 (the section B in FIG. 10). The structure of the reference voltage generating circuit 108 and the selector circuit 106 shown in FIG. 10 is arranged such that the structural pattern shown in FIG. 11 are repeated.
The reference voltage generating circuit 108 generates a plurality of reference voltages (64 reference voltages in this example) in accordance with the display data provided in the form of digital signals. The selector circuit 106, which is made up of MOS transistor switches, selects one of the reference voltages for output. The structure of the switches will be described later in detail.
The voltage follower circuit 107 outputs the voltage selected by the selector circuit 106 through a liquid crystal driving voltage output terminal (corresponding to a liquid crystal driving voltage output terminal 17t in FIG. 9) to a liquid crystal display element,; as a liquid crystal driving signal.
Generally, the reference voltage generating circuit 108 is commonly used for a plurality of liquid crystal driving voltage output terminals.
Meanwhile, one selector circuit 106 and one voltage follower circuit 107 are provided for each liquid crystal driving voltage output terminal. In the case of a color display, a liquid crystal driving voltage output terminal is used corresponding to each color, thus one selector circuit 106 and one voltage follower circuit 107 are provided for each color in a pixel. That is, supposing that the total number of pixels in a liquid crystal panel (corresponding to a liquid crystal panel 21 in FIG. 5) is N, and liquid crystal driving voltage output terminals for red, green, and blue are expressed as R, G, and B, respectively with a subscript n (n=1, 2, . . . , N), the liquid crystal panel has liquid crystal driving voltage output terminals R1, G1, B1, R2, G2, B2, . . . ) RN, GN, BN, therefore 3N selector circuits 106 and 3N voltage follower circuits 107 are required.
The following description will explain the structure and operation of the DA converter 100 incorporated ;in the liquid crystal driving device.
The reference voltage generating circuit 108 is structured such that 64 resistor elements are connected in series, and a largest liquid crystal driving voltage V64 and a smallest liquid crystal driving voltage V0 are inputted to the respective end terminals. Therefore, 64 voltages (V0 to V63) are generated respectively from between the resistor elements, in proportion to the resistance of the resistor elements connected. These 64 different voltages generated from the reference voltage generating circuit 108 are inputted to the selector circuit 106.
In the selector circuit 106, the MOS transistor switches are arranged so as to select one of the 64 inputted voltages for output in accordance with the display data composed of a 6-bit digital signal. That is, the switches are turned on or off in response to each piece of display data composed of a 6-bit digital signal (Bit 0 to Bit 5), thus one of the 64 inputted voltages is selected for output. The following description will explain voltage selection procedures in detail.
In a 6-bit digital signal, Bit 5 is MSB (Most Significant Bit), and Bit 0 is LSB (least Significant Bit). The switches are paired in two to form switch pairs. Therefore, Bit 0 is provided with 32 switch pairs (64 switches), and Bit 1 is provided with 16 switch pairs (32 switches). AS moving up from a certain bit to a next bit, the number of switch pairs decreases by half, down to a single switch pair (two switches) for Bit 5. So the total number of the switch pairs in the selector circuit 106 amounts to 1+2+22+23+24+25=63 (126 switches).
The two switches of a switch pair operate such that, if the corresponding bit is xe2x80x9c0xe2x80x9d, the upper switch in the figure turns off, and the lower switch turns on. In contrast, if the corresponding bit is xe2x80x9c1xe2x80x9d, the upper switch turns on, and the lower switch turns off. For example, referring to the example in FIG. 12, Bit 5, Bit 4, . . . , Bit 0 is xe2x80x9c111111xe2x80x9d, so all the upper switches are on and all the lower switches are off, allowing the voltage V63 to be outputted from an output terminal of the selector circuit 106. Further, for example, if Bit 5, Bit 4, . . . , Bit 0 is xe2x80x9c000001xe2x80x9d, the voltage V1 is outputted from an output terminal of the selector circuit 106.
The voltage follower circuit 107 provides a voltage identical to the analog voltage outputted from the selector circuit 106, for output via the liquid crystal driving voltage output terminal as a liquid crystal driving signal having a smaller internal resistance.
There is a trend in recent years for liquid crystal display devices to include more minute structure and display more gradation levels, resulting in increases in the size of circuits in liquid crystal driving devices. Meanwhile, as liquid crystal display devices find applications in more fields, there are increasingly higher demands for cheaper liquid crystal display devices and stronger needs to reduce manufacturing costs by manufacturing smaller liquid crystal driving devices. Further, in terms of portability, there are strong demands for smaller liquid crystal display devices including liquid crystal driving devices, which adds the importance of the reduction in the size of liquid crystal driving devices.
However, when the foregoing conventional DA converter 100 is used as a liquid crystal driving device of a liquid crystal display device, the number of circuit composing elements increases sharply in accordance with an increase in the number of the gradation levels to be displayed. For example, in the case of a liquid crystal driving device providing a 64-level gradation display using 6-bit digital signals, 64 resistor elements are required in the reference voltage generating circuit 108, and 126 switches are required for every pixel to form the selector circuit 106. In the same manner, in the case of a liquid crystal driving device providing a 256-level gradation display using 8-bit digital signals, 256 resistor elements are required in the reference voltage generating circuit 108, and 510 switches are required for every pixel to form the selector circuit 106, as it includes 1+2+22+23+ . . . +27=255 switch pairs. Further, when performing a color display, since there are three primary colors (red, green, and blue), the number of the resistor elements and switches required triples.
In this manner, the liquid crystal driving device in accordance with conventional technology requires an increasingly large number of circuit composing elements according to the increase in the number of the colors to be displayed, that is, the increase in the number of voltages required to provide more gradation levels. In addition, a higher definition display requires greatly larger number of the circuit composing elements. Therefore, the cost for manufacturing the liquid crystal driving display sharply increases, and the chip size grows when the liquid crystal driving device is fabricated as an integrated circuit, which renders it difficult to reduce the size.
The present invention has an object to offer a DA converter which restrains sharp increases in the number of circuit composing elements (resistor elements and switches) despite a possible increase in the number of voltages required, therefore suppressing increases in manufacturing cost and being provided in a more compact size.
The present invention has another object to offer a liquid crystal driving device which restrains large increases in the number of circuit composing elements despite a display with more gradation levels and higher definition, therefore suppressing increases in manufacturing cost and being provided in a more compact size.
In order to achieve the objects, a DA converter in accordance with the present invention includes:
a reference voltage generating circuit for generating the reference voltages;
a reference voltage selector circuit for selecting two reference voltages having adjacent voltage levels so as to include the voltage level of an output voltage between the voltage levels of the two reference voltages;
an output voltage selector circuit for selecting the voltage level of the output voltage from a plurality of voltage levels predetermined between the voltage levels of the two reference voltages; and
a voltage generating circuit for generating a voltage having the voltage level selected by the output voltage selector circuit as the output voltage, based on the two reference voltages.
According to the foregoing structure, the DA converter generates interpolation voltages which: have voltage levels not generated by the reference voltage generating circuit, based on the reference voltages generated by the reference voltage generating circuit. As a result, the DA converter can output voltages not generated by the reference voltage generating circuit (interpolation voltages) as well as the voltages generated by the reference voltage generating ciircuit (reference voltages) as the output voltages. Further, in the DA converter, the voltage level of an interpolation voltage can be selected from a plurality of voltage levels predetermined between adjacent reference voltages.
Consequently, in the DA converter, a part of the voltage levels required to provide an output voltage can be generated by interpolation, permitting the reduction in the number of voltages generated by the reference voltage generating circuit less than the number required. Therefore, the number of elements in the reference voltage generating circuit and in the reference voltage selector circuit can be greatly reduced in comparison with conventional technology.
Therefore, despite a possible increase in the number of voltages required, large increases in the number of circuit composing elements such as resistor elements and switches can be avoided, thus preventing an increase in manufacturing cost and permitting the production of more compact devices.
Further, a liquid crystal driving device in accordance with the present invention is arranged so as to include the DA converter.
With this arrangement, the liquid crystal driving device can restrain large increases in the number of circuit composing elements despite a display with more gradation levels and higher definition, therefore suppressing increases in manufacturing cost and being provided in a more compact size.
Therefore, the reduction in the size of the reference voltage selector circuit, which occupies a large size in a liquid crystal driving circuit, permits a great reduction in chip size and substantially contributes to the resulting cost reduction. In addition, the reduction in the size of the circuit in a liquid crystal driving device allows a liquid crystal display device incorporating the liquid crystal driving device to be provided in a more compact size. Therefore, high quality display devices with higher resolution and more gradation display capabilities using increasingly large number of pixels can be realized, as well as more compact display devices can be provided.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.